The instant invention relates to improved photoreceptors for use in electrophotographic imaging processes. The photoreceptors of the instant invention are characterized by (1) increased charging potential (saturation voltage, V.sub.sat) as compared to prior art photoreceptors (2) substantially decreased loss of stored charge with the passage of time (dark decay) and (3) a decreased tendency of the component layers to crack and peel.
Electrophotography, also referred to generically as xerography, is an imaging process which relies upon the storage and discharge of an electrostatic charge by a photoconductive material for its operation. A photoconductive material is one which becomes electrically conductive in response to the absorption of illumination; i.e., light incident thereupon generates electron-hole pairs (referred to generally as "charge carriers"), within the bulk of the photoconductive material. It is these charge carriers which permit the passage of an electrical current through that material for discharge of the static electrical charge stored thereupon.
First the structure and then the operation of a typical xerographic or electrophotographic photoreceptor will be explained so that the operation and advantages of the instant invention may be fully appreciated.
As to the structure: A typical photoreceptor includes a cylindrical, electrically conductive substrate member, generally formed of a metal such as aluminum. Other substrate configurations, such as planar sheets, curved sheets or metallized flexible belts may likewise be employed. The photoreceptor also includes a photoconductive layer, which as previously described, is formed of a material having a relatively low electrical conductivity in the dark and a relatively high electrical conductivity under illumination. Disposed between the photoconductor and the substrate is a blocking layer, formed either by the oxide naturally occuring on the substrate, or from a deposited semiconductor layer. As will be discussed in greater detail hereinbelow, the blocking layer functions to prevent the flow of unwanted charge carriers from the substrate into the photoconductive layer where they could then neutralize the charge stored upon top surface of the photoreceptor. A typical photoreceptor also generally includes a top protective layer disposed upon the photocoductive layer to stabilize the electrostatic charge acceptance against changes due to adsorbed chemical species and to improve the photoreceptor durability.
In operation of the electrophotographic process: the photoreceptor must first be electrostatically charged in the dark. Charging is typically accomplished by a corona discharge or some other such conventional source of static electricity. An image of the object to be photographed, for example a typewritten page, is then projected onto the surface of the charged electrophotographic photoreceptor. Illuminated portions of the photoconductive layer, corresponding to the light areas of the projected image, become electrically conductive and pass the electrostatic charge residing thereupon through to the electrically conductive substrate thereunder which is generally maintained at ground potential. The unilluminated or weakly illuminated portions of the photoconductive layer remain electrically resistive and therefore continue to be proportionally resistive to the passage of electrical charge to the grounded substrate. Upon termination of the illumination, a latent electrostatic image remains upon the photoreceptor for a finite length of time (the dark decay time period). This latent image is formed by regions of high electrostatic charge (corresponding to dark portions of the projected image) and regions of reduced electrostatic charge (corresponding to light portions of the projected image).
In the next step of the electrophotographic process a fine powdered pigment bearing an appropriate electrostatic charge and generally referred to as a toner, is applied (as by cascading) onto the top surface of the photoreceptor where it adheres to portions thereof which carry the high electrostatic charge. In this manner a pattern is formed upon the top surface of the photoreceptor, said pattern corresponding to the projected image. In a subsequent step the toner is electrostatically attracted and thereby made to adhere to a charged receptor sheet which is typically a sheet of paper or polyester. An image formed of particles of toner material and corresponding to the projected image is thus formed upon the receptor sheet. In order to fix this image, heat and/or pressure is applied while the toner particles remain attracted to the receptor sheet. The foregoing describes a process which is the basis of many commercial systems, such as plain paper copiers and xeroradiographic systems.
It should be clear from the foregoing discussion that the electrophotograhic photoreceptor represents a very important element of the imaging apparatus. In order to obtain high resolution copies, it is desirable that the photoreceptor accept and retain a high static electrical charge in the dark; it must also provide for the flow of that charge from portions of the photoreceptor to the grounded substrate under illumination; and it must retain substantially all of the initial charge for an appropriate period of time in the non-illuminated portions without substantial decay thereof.
Image-wise discharge of the photoreceptor occurs through the photoconductive process previously described. However, unwanted discharge may occur via charge injection at the top or bottom surface and/or through bulk thermal charge carrier generation in the photoconductor material.
A major source of charge injection is at the metal substrate/semiconductor interface. The metal substrate provides a virtual sea of electrons available for injection and subsequent neutralization of, for example, the positive static charge on the surface of the photoreceptor. In the absence of any impediment, these electrons would immediately flow into the photoconductive layer; accordingly, all practical electrophotgraphic media include a bottom blocking layer disposed between the substrate and the photoconductive member. This bottom blocking layer is particularly important for electrophotographic devices which employ photoconductors with dark conductivities greater than 10.sup.-13 ohm.sup.-1 cm.sup.-1. As mentioned hereinabove, in some cases the blocking layer may be formed by native oxides occuring upon the surface of the substrate, as for example a layer of alumina occuring on aluminum. In other cases, the blocking layer is formed by chemically treating the surface of the substrate. Since it is practically important to the electrophotographic copying process to have unipolar charging characteristics, an important class of blocking layers is formed by depositing a layer of semiconductor alloy material of appropriate conductivity type onto the substrate to give rise to substantially diode-like blocking conditions.
In order to better understand the manner in which the blocking layers operate, it is necessary to review in greater depth a portion of the physics involved in the blocking layer phenomenon. As previously mentioned, the blocking layer must inhibit the transport and subsequent injection of the appropriate charge carrier (electrons for a positively charged drum) principally from the metal substrate into the body of the photoreceptor. This is accomplished in the doped semiconductor blocking layer by establishing a condition in which the minority charge carrier drift range, mu tau E, is smaller than the blocking layer thickness. Here, mu is the minority carrier mobility, tau is the minority carrier lifetime and E is the electric field strength. One can, for instance, substantially reduce the mu tau product for electrons by doping the blocking layer p-type. The excess holes present in the doped blocking layer greatly increase the probability of electron-hole recombination, thereby reducing the electron lifetime, tau. In effect one achieves a condition whereby electrons injected from the metal substrate recombine with holes in the p-type blocking layer before they are able to drift into the bulk of the photoreceptor to be swept through the top surface and neutralize the static charge thereon. However, while doping can serve to limit the mu tau product for the desired carrier, it can also give rise to deep electronic energy levels in the semiconductor alloy material. This is particularly true for semiconductors such as amorphous silicon alloys where the efficiency of substitutional doping is not high. These deep levels can become the source of thermally generated carriers or they can, if sufficiently numerous, provide a parallel path for the hopping conduction of electrons through the doped layers. Either of these phonomena can serve to compromise the blocking function of the doped layers.
Amorphous silicon alloys have great utility as photoconductors insofar as they manifest excellent bipolar photoconductivity, are durable, non-toxic and can be economically fabricated (in view of the disclosure regarding the use of microwave frequencies found in commonly assigned U.S. Pat. No. 4,504,518). However due to the short dielectric relaxation time of these photoconductors, the electrophotographic utility of amorphous silicon alloys relies heavily upon high quality blocking layers used in combination therewith.
One approach to the problem of fabricating barrier layers is disclosed in U.S. Pat. No. 4,378,417 of Maruyama, et al entitled "Electrophotographic Member With a-Si Layers." As disclosed in Maruyama, et al, a barrier layer formed of deposited oxides, sulfides or selenides may be utilized to prevent the injection of charge carriers into an amorphous silicon photoconductive layer.
Fukuda, et al in U.S. Pat. No. 4,359,512 entitled "Layered Photoconductive Member Having Barrier of Silicon and Halogen" disclose a barrier layer formed of an amorphous silicon:hydrogen:halogen alloy. A similar approach is reported in more detail in a paper entitled "Photoreceptor of a-Si:H With Diodelike Structure for Electrophotography" by Isamu Shimizu et al, published in J. Appl. Phys. 52 (4), April 1981, pp 2776-2781.
Shimizu, et al disclose doped amorphous silicon barrier layers for use in amorphous silicon photoreceptors. The data of Shimizu, et al gives a good illustration of the aforementioned need to compromise between the prevention of charge injection and the initiation of hopping conduction. FIG. 3a of Shimizu, et al graphically represents the change in saturation voltage (i.e. maximum charging voltage) of a photoreceptor as a function of increasing p-doping of the amorphous silicon barrier layer thereof. It will be noted from an inspection of the Figure that, with an essentially undoped blocking layer, the photoreceptor achieves a charge acceptance of approximately 35 volts per micron. As the level of doping is increased, the charge acceptance increases up to a maximum value of approximately 50 volts per micron (for a two micron laboratory sample) attained at a diborane doping level of approximately 360 ppm in the process gas. Further increases in the doping levels only serve to decrease the charge acceptance.
The initial rise in the charge acceptance results from a decrease in the mu tau product for electrons with increasing boron doping and is indicative of the increasing efficiency with which the blocking layer prevents charge injection. However the subsequent fall off in efficiency results from the onset of electron hopping conduction in the increasingly heavily doped, highly defective blocking layer. Note that the blocking layer becomes highly defective because the incorporation of the boron dopant into the host matrix of the amorphous silicon alloy material of that layer is not completely substitutional; that is to say, many of the dopant atoms do not directly substitute for silicon atoms in the amorphous matrix, but rather alloy or otherwise insert themselves in a manner which produces defect states.
Referring to FIG. 1 of Shimizu, et al it may be ascertained that at the 360 ppm doping level, the Fermi level of the resultant p-doped alloy is approximately 0.6 eV from the valence band. As is readily apparent to one skilled in the art, a higher degree of blocking would be obtained if one could employ a more heavily p-doped alloy from which to form the blocking layer. This more heavily doped blocking layer would produce an even smaller electron mu tau product and consequently provide even more effective inhibition of electron transport through the blocking layer. However, as is apparent from the data presented, Shimizu, et al were unable to employ such a more heavily doped alloy because of the inherent problem of electron hopping initiated by the doping-induced defect states. As will be noted from FIG. 3b thereof, the maximum charging voltage obtained by Shimizu, el al (in a photoreceptor approximating commercial utility) was slightly under 400 volts for a photoconductive layer 10 microns thick. This represents a charge acceptance of just under 40 volts per micron.
As mentioned previously, it is highly desirable to provide a blocking layer of optimized efficiency. All other properties being kept constant, a photoreceptor having an efficient blocking layer will manifest a higher saturation voltage and therefore will produce higher contrast copies than a photoreceptor having a less efficient blocking layer. Alternatively, a photoreceptor with high charge acceptance can be made thinner while still achieving the same saturation voltage thus reducing manufacturing costs through savings in fabrication time and materials costs. Additionally, a more efficient blocking layer may be made thinner, thereby decreasing stress in the deposited layers (a thinner photoreceptor is inherently less stressed), which stress can result in cracking and peeling of the layers thereof. Furthermore, the use of a highly efficent blocking layer would allow the incorporation of lower quality photoconductive material into an electrophotographic photoreceptor (a plus in production since it is easier and faster to fabricate poorer material), insofar as losses resulting from the poor quality material would be offset by gains made through the use of the more efficient blocking layer.
The instant invention provides for highly efficient blocking layers through the fabrication of those layers from highly conductive microcrystalline semiconductor alloy material. In light of the many definitions utilized for the terms "amorphous" and "microcrystalline" in the scientific and patent literature it will be helpful to clarify the definition of those terms as used herein.
The term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. As used herein the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occurs. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, materials employed in the practice of the instant invention fall within the generic term "amorphous" as defined hereinabove.
The concept of microcrystalline materials exhibiting a threshold volume fraction of crystalline inclusions at which substantial charges in key parameters occur, can be best understood with reference to the percolation model of disordered materials. Percolation theory, as applied to microcrystalline disordered materials, analogizes properties such as the electrical conductivity manifested by microcrystalline materials, to the percolation of a fluid through a non-homogeneous, semi-permeable medium such as a gravel bed.
Microcrystalline materials are formed of a random network which includes low mobility, highly disordered regions of material surrounding randomized, highly ordered crystalline inclusions or grains having high carrier mobility. Once these crystalline inclusions attain a critical volume fraction of the network, (which critical volume will depend, inter alia, upon the size and/or shape and/or orientation of the inclusions), it becomes a statistical probability that said inclusions are sufficiently interconnected so as to provide a low resistance current path through the network. Therefore at this critical or threshold volume fraction, the material exhibits a sudden increase in conductivity. This analysis (as described in general terms relative to electrical conductivity herein) is well known to those skilled in solid state theory and may be similarly applied to describe additional physical properties of microcrystalline materials, such as optical gap, absorption constant, etc.
The onset of this critical threshold value for the substantial change in physical properties of microcrystalline materials will depend upon the size, shape and orientation of the particular crystalline inclusions, but is relatively constant for different types of materials. It should be noted that while many materials may be broadly classified as "microcrystalline" those materials will not exhibit the properties we have found advantageous for the practice of our invention unless they have a volume fraction of crystalline inclusions which exceeds the threshold value necessary for substantial change. Accordingly we have defined "microcrystalline materials" to include only those materials which have reached the threshold value. Further note that the shape of the crystalline inclusions is critical to the volume fraction necessary to reach the threshold value. There exist 1-D, 2-D and 3-D models which predict the volume fraction of inclusions necessary to reach the threshold value, these models being dependent on the shape of the crystalline inclusions. For instance, in a 1-D model (which may be analogized to the flow of charge carriers through a thin wire), the volume fraction of inclusions in the amorphous network must be 100% to reach the threshold value. In the 2-D model (which may be viewed as substantially conically shaped inclusions extending through the thickness of the amorphous network), the volume fraction of inclusions in the amorphous network must be about 45% to reach the threshold value. And finally in the 3-D model (which may be viewed as substantially spherically shaped inclusions in a sea of amorphous material), the volume fraction of inclusions need only be about 16-19% to reach the threshold value. Therefore, amorphous materials (even materials classified as microcrystalline by others in the field) may include crystalline inclusions without being microcrystalline as that term is defined herein.
Accordingly, the amorphous materials of Maruyama and Shimizu are differentiated from the microcrystalline materials of the instant invention although all may be broadly and generically termed "amorphous".
As will be described in greater detail hereinbelow, the blocking layers of the instant invention are highly efficient insofar as a high degree of substitutional doping may be readily attained therein. The greater the degree of substitutional doping, the more effectively the minority carrier mu tau product can be reduced while producing fewer defect sites which promote the hopping conduction of electrons. Furthermore, since the highly doped microcrystalline blocking layers of the instant invention are of high electrical conductivity; the large density of free charge carriers can move so as to effectively screen the electric field, E, in the blocking layer when the photoreceptor is charged. This reduced electric field produces a drift range (mu-tau-E) which is very small. Due to the microcrystalline nature of the semiconductor blocking layers of the instant invention, said layers may be doped to the point of electrical degeneracy, i.e., the Fermi level is essentially coincident with the majority carrier band edge. This has the effect of causing the activation energy for the thermal generation of unwanted minority carriers to be the maximum possible value, i.e. the semiconductor band gap energy. This is to be contrasted with prior art blocking layers, such as described in Shimizu, et al, which could not be heavily doped without providing defect sites which rendered their blocking layers practically useless through the mechanisms of thermal generation and/or hopping. Further, and as previously mentioned, the optimal doping for Shimizu, et al's blocking layer resulted in a Fermi level position about 0.6 eV away from the appropriate band edge. Therefore, the conductivity of that blocking layer remained relatively low so as to ineffectively screen the electric field, E, in the blocking layer when the photoreceptor is charged. Of course, the high electric field then produces a relatively high drift range (mu-tau-E), which high drift range allows electrons injected from the metal substrate to drift through the blocking layer and neutralize static charge on the top surface of the photoreceptor. Furthermore, since Shimizu, et al cannot lower the activation energy of their material below 0.6 Ev without compromising the efficacy of their blocking layer, their photoreceptors will exhibit a high degree of thermal charge carrier generation from the Fermi level at the blocking layer/photoconductor interface. Since the microcrystalline materials described herein may be readily doped to degeneracy, they present as previously mentioned, the highest possible barrier (at the blocking layer/photoconductor interface) to the thermal generation of carriers from states located at the Fermi level.
By employing the principles of the instant invention, electrophotographic photoreceptors having highly efficient, highly doped blocking layers may be readily fabricated. Since the blocking layers are microcrystalline, they show less internal stress. And since the blocking layers are so efficient the overall photoreceptor thickness may be reduced, providing substantial reduction in manufacturing cost, decreased internal stress and a consequent decreased tendency towards cracking and peeling.
It is important to note that conventional scientific wisdom was diametrically opposed to experimenting with the use of highly doped microcrystalline material from which to fabricate the blocking layers for photoelectric photoreceptors. From a purely empirical point of view, the results published by Shimizu, et al taught away from increasing the doping concentration, and consequently the blocking layer conductivity, above the values obtained at approximately 350 ppm gas phase ratio of B.sub.2 H.sub.6 to SiH.sub.4. Further, other experience taught away from employing microcrystalline material since it was anticipated that these materials would exhibit such a high volume percentage of grain boundaries and attendent defects as to cause hopping conduction of charge carriers, thereby compromising the blocking function and providing for the neutralization of the surface charge of the photoreceptor. It was for this reason that Applicants, in commonly assigned patent application Ser. No. 580,081 filed Feb. 14, 1984, stated " . . . the bottom blocking layer does not have to be amorphous and can be, for example, polycrystalline . . . ". However, because Applicants believed the grain boundaries to be so defective as to cause hopping conduction at the Fermi level, they did not include microcrystalline material as a possible candidate from which to fabricate said bottom blocking layer.
However, it was synergistically discovered that the microcrystalline material described hereinabove was characterized by grains of sufficiently large size that the surface state defects on grain boundaries did not promote substantial hopping conduction through the blocking layer and into the bulk of the photoreceptor. For purposes of this definition microcrystalline material will be referred to as having grains under approximately 5000 Angstroms thickness and the polycrystalline material referred to in said patent application Ser. No. 580,081 has grains from approximately 5000 Angstroms to monocrystalline. Regardless of the reason for the surprising performance of the microcrystalline blocking layer, experiments have clearly demonstrated the vastly improved results in photoreceptors made possible through the use of these microcrystalline blocking layers. Specifically, saturation voltages in 20 micron thick photoreceptors which included a microcrystalline blocking layer were as high as 1296 volts with dark decay ratios (ratio of charge remaining to initial charge after three seconds of discharge) as high as 0.7. This represents a marked improvement over otherwise identically prepared 20 micron thick photoreceptors which included an optimally doped amorphous blocking layer, said latter photoreceptors characterized by saturation voltages only as high as 582 volts and dark decay ratios of 0.5.
Further, and as will be discussed in greater detail hereinbelow, the blocking layers of the instant invention may be readily fabricated from a wide variety of semiconductor materials by rapid, economical, easy to implement deposition processes.
These and other objects and advantages of the instant invention will be apparent from the detailed description of the invention, the brief description of the drawings and the claims which follow.